It really depends on what your building, how quickly you need to build it, and what the client cares about. It could be no-one cares if MyServiceClass has a sma

It really depends on what your building, how quickly you need to build it, and what the client cares about. It could be no-one cares if MyServiceClass has a small bug that causes it to crash and restart from time to time - I worked in a startup that had something like that and it was fine within the needs of the company. Nowadays I work on financial stuff and a crash could cost ££££ so it's unacceptable, meaning we need more testing to check stuff. Also with interfaces, they matter more if your working in a large distributed team. Sometimes you need to have the method signatures designed upfront so other teams can work on it, meaning it's less trivial than say an internal subroutine used in a private class.